Acquisitions and Dispositions (Narrative) (Details) $ in Millions		3 Months Ended				6 Months Ended		12 Months Ended
	Jan. 01, 2024 USD ($)	Jun. 30, 2024 USD ($)	Mar. 31, 2024 USD ($) acquisition	Jun. 30, 2023 USD ($)	Mar. 31, 2023 USD ($)	Jun. 30, 2024 USD ($) acquisition	Jun. 30, 2023 USD ($)	Dec. 31, 2023 acquisition
Business Acquisition [Line Items]
Total consideration						$ 716	$ 340
Cash paid						662	332
Estimated fair value of deferred/contingent purchase consideration						54	8
Deferred purchase consideration from prior years' acquisitions						10	51
Contingent consideration from prior year's acquisitions						71	134
Cash divested from deconsolidation					$ 20
Purchase of non-controlling interests				$ 139		0	139
Total Segments | Small Individual Financial Advisory Business
Business Acquisition [Line Items]
Net gain (loss) on disposition of business							(17)
Discontinued Operations, Disposed of by Sale | U.S. Health And Benefits And U.K Pension Administration Businesses
Business Acquisition [Line Items]
Cash proceeds from sale	$ 114
Net gain (loss) on disposition of business	$ 21
Exit costs			$ 18
Discontinued Operations, Disposed of by Sale | U.S. Health And Benefits And U.K Pension Administration Businesses | Total Segments
Business Acquisition [Line Items]
Net gain (loss) on disposition of business						21
Prior Fiscal Periods Acquisitions
Business Acquisition [Line Items]
Deferred purchase consideration from prior years' acquisitions							51
Contingent consideration from prior year's acquisitions						161	175
Pro forma revenue				41			41
Operating income (loss) related to acquisitions				9			9
Current Fiscal Period Acquisitions
Business Acquisition [Line Items]
Total consideration						716
Cash paid						662
Estimated fair value of deferred/contingent purchase consideration						54
Pro forma revenue		$ 48				62
Operating income (loss) related to acquisitions		3				(3)
Westpac Transaction
Business Acquisition [Line Items]
Acquisition related costs		$ 21		$ 10		$ 29	$ 27
Minimum
Business Acquisition [Line Items]
Revenue target period (in years)						2 years	2 years
Maximum
Business Acquisition [Line Items]
Revenue target period (in years)						4 years	4 years
Risk and Insurance Services
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions) | acquisition						5		9
Consulting
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions) | acquisition			5			4